Synaptic organization of the cerebral cortex including the primary visual cortex and the hippocampal formation in rats and monkeys will be studied with electron microscopy, Golgi preparations, and autoradiography. Nerve cells and nerve endings in cortex will be classified according to their fine structural characteristics and attempts will be made to correlate these classes with putative neurotransmitters. Nerve endings will be identified according to their nerve cells of origin and their locations. Such an analysis should result in an understanding of the design of these two regions of cerebral cortex, which may then form the basis for the construction of plausible neuronal circuits that can be tested physiologically.